US3505477A - Impedance network for resonant transfer multiplexing - Google Patents

Impedance network for resonant transfer multiplexing Download PDF

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Publication number
US3505477A
US3505477A US624530A US62453067A US3505477A US 3505477 A US3505477 A US 3505477A US 624530 A US624530 A US 624530A US 62453067 A US62453067 A US 62453067A US 3505477 A US3505477 A US 3505477A
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transformer
filter
inductance
impedance
band
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Alfred Leo Maria Fettweis
Joseph Antonius Broux
Charles Gerard Cornelius Cool
Robert Pierre Emile Fer Salade
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Alcatel Lucent NV
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International Standard Electric Corp
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Assigned to ALCATEL N.V., DE LAIRESSESTRAAT 153, 1075 HK AMSTERDAM, THE NETHERLANDS, A CORP OF THE NETHERLANDS reassignment ALCATEL N.V., DE LAIRESSESTRAAT 153, 1075 HK AMSTERDAM, THE NETHERLANDS, A CORP OF THE NETHERLANDS ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: INTERNATIONAL STANDARD ELECTRIC CORPORATION, A CORP OF DE
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J3/00—Time-division multiplex systems
    • H04J3/20—Time-division multiplex systems using resonant transfer

Definitions

  • An impedance network of this kind is essentially composed of reactive elements constituting a low pass filter to be used in a time division multiplex system wherein it serves to convert the pulse amplitude modulated energy into the voice frequency band.
  • a time division multiplex system it is usually desirable to operate the pulse system on an unbalanced basis, this in order to avoid a duplication of the gates involved in such systems.
  • the subscriber lines and other terminations, such as junctions are balanced circuits whereby it is essential to provide a transformer between the low pass filter and the balanced line.
  • At least one other reason why such a transformer is essential is that the control circuits for the transistors which are used as gates must be isolated from the external accesses.
  • a step-up ratio from the balanced line to the unbalanced low pass filter is desirable in order to raise the impedance level of the filter so that the switching current through the gating transistor may be limited, the step-up ratio being however chosen low enough so that the voltage applied to this gating transistor will not exceed the limits thereof.
  • the low pass filter offers a capacitive impedance at infinite frequency and a small inductance is inserted in series with the transistor gate with such a value that it resonates with the equivalent capacitance of the low pass filter, at a frequency such that the corresponding half period is equal to the time during which the transistor gate is unblocked, this time being only a small fraction of the sampling period.
  • a theoretically perfect exchange of energy can take place between the equivalent storage capacitances of the two low pass filters and a practically lossless transmission can be obtained.
  • this transformer should not modify the response of the optimized filter.
  • ideal transformers are useful devices for the analysis of transmission networks, in practice one has to contend with ordinary transformers which inevitably introduce spurious elements in the transmission path since their shunt inductance is not ideally infinite and their leakage inductance is different from zero.
  • its reactance should be at least equal to three times the terminating resistance on the corresponding side of the transformer and obviously, the larger it is, the lower will be the additional loss, especially at the lower end of the passband.
  • the microphone current supply must be decoupled from the speech path with the help of a capacitor, usually of 2 microfarads, and in order to keep a balanced line circuit, the primary winding of the transformer on the line side is split to enable the insertion of this capacitor.
  • the latter constitutes a second disturbing element since it cannot be made of sufiiciently high value not to adversely affect the transmission characteristics.
  • the general object of the invention is to obtain an improved electrical transmission system of the type discussed above in which the spurious effects of such elements are eliminated.
  • the invention is based on the insight that unexpectedly, instead of using larger values for such elements as the line transformer and the splitting capacitor much smaller values than those usually taken as minima by the designer may be found to result in a vary substantially improved transmission characteristic.
  • an impedance network as initially defined is characterised in that said coupled inductances constitute a shunt inductive impedance in the transmission path whose value at the lowest frequency to be transmitted and when measured on the side of said resistive termination is lower than three times the impedance of-said resistive termination.
  • said coupled inductance on the side of said resistive termination is coupled thereto through a series capacitance.
  • said coupled inductances with the series capacitance connected on the side of the resistive termination and said impedance network together constitute a bandpass filter.
  • said band-pass filter is antimetrical, i.e., having inverse image impedances.
  • Such a structure can be found to make possible the inclusion of a practical transformer and a splitting capacitor in the design, while securing an adequately low value for the shunt inductance corresponding to the transformer so that the reduced value of the winding resistances means a much lower attenuation in the pass-band.
  • a symmetrical band-pass filter on the other hand could not take into account the leakage inductance of the practical transformer nor the DC. splitting capacitor and moreover, a computation of such a symmetrical band-pass filter leads to an equivalent shunt inductance whose value is more than times that of the shunt inductance secured with an antimetrical design.
  • FIG. 2 the electrical transmission network of the instant invention
  • FIG. 3 the image impedance function of a filter
  • FIG. 4 the envelope characteristic of the attenuation function of the pass-band filter within the pass-band
  • FIG. 5 a particular design of part of the circuit of FIG. 2;
  • FIG. 6 an attenuation curve showing the improvements obtained by the filter designed in accordance with the present invention.
  • FIG. 1 shows a time division multiplex resonant transfer transmission circuit of the type disclosed in the US. Patent No. 3,187,101 previously mentioned.
  • a telephone substation represented "by a source of electrical signals E and a source resistance R may be connected by a balanced transmission line to a line transformer TR whose split primary windings are interconnected by a DC. splitting capacitor C whose plates are coupled to ground and negative battery through resistances R and R respectively, through which resistances can flow the DC. current needed by the carbon microphone (not shown) at the substation.
  • Call detecting elements may be coupled at the junction of R and C and are not shown here as they have no bearing on the invention.
  • the secondary side of TR is unbalanced, one end of the winding being coupled to a central ground through the outer conductor of a coaxial cable CC which constitutes a common highway used by a group of e.g. 100 substations in time division multiplex fashion as indicated by the multiplying arrow A.
  • LPF low pass filter
  • LPF the low pass filter
  • LPF is coupled to the inner conductor of the CC through the emitter path of a PNP transistor T serving as electronic gate in series with a small inductance L constituting the resonant transfer inductance.
  • the base of T is shown to be coupled to the ground conductor through control network CTN which will not be further described as it may for instance be of the type disclosed in the above mentioned band-pass US. Patent No.
  • 3,187,101 serves to regularly unblock transistor T, say during time intervals of two microseconds out of a channel time of four microseconds and at a sampling frequency of 10 kc./s. corresponding to a sampling period of microseconds. This 100 microsecond period is large with respect to the time during which the connection is established.
  • the coaxial conductor CC may be coupled through a further gating transistor not shown to a central capacitance CH which in turn may be connected through exactly identical circuits to another substation in the network, e.g. through the coaxial conductor CC serving as highway for the group of that second substation (not shown).
  • FIG. 2 shows a circuit which serves to explain the transmission properties of the network of the present invention.
  • transformer TR has been replaced by the shunt inductance L the series inductance L and which corresponds to the leakage inductance of the transformer, the third element thereof being the ideal transformer of voltage step-up ratio l/n which is inserted between shunt inductance L and shunt capacitance C
  • the feeding resistances R and R of FIG. 1 play no part in the transmission properties and accordingly have been omitted from FIG. 2 wherein capacitance C is shown in series between L and R.
  • coaxial cable CC and CC shown in FIG. 1 have been represented in FIG. 2 by simplified circuits each consisting of a series inductance such as LC and a shunt capacitance CC, the first being added in series with LT to constitute the overall resonant transfer inductance L, and the second being added in shunt across capacitance CH together with the like shunt capacitance CC of the other coaxial cable CC, to form an overall central shunt capacitance which is adjusted with the help of CH to a value 2C/ 3.
  • a series inductance such as LC and a shunt capacitance CC
  • the low pass filter LPF of FIG. 1 may be designed on an open-circuit basis by classical methods since transistor T is only made conductive during a very short part of the sampling period. It will now be shown that the network BPF of FIG. 2 incorporating the transformer TR and the capacitor C of FIG. 1 constitutes a suitable band-pass filter of the antimetrical type which may be designed in a suitable manner, for instance by using image impedance theory.
  • the image impedance of the band-pass filter considered here is one of the lowest class and particularly the Z'n' type of image impedance or its inverse of the ZT type considered for instance in the Belgian Patent No. 624,163 or in Revue HF, 1963, No. 11, Image parameter and effective loss design of symmetrical and antimetrical crystal band-pass filters, A. Fettweis, which are given by:
  • x is a normalized frequency variable given by f m wherein f is the frequency and f the mid-band frequency of the band-pass filter which is equal to the geometric mean of the theoretical lower and upper cut-off frequency f, and f respectively, m being a dimensionless parameter defined by i i fm fa and b being the relative bandwidth, i.e.
  • a is the minimum value of the normalized Z1r/R image impedance which is obtained when x is equal to unity as shown on FIG. 3.
  • the filter such as BPF shown in FIG. 2 should be designed as open-circuit filter by virtue of its being used in a time division multiplex transmission network using the gating transistor T and as already pointed out in the Belgian Patent No. 606,649 in connection with an improved design considering resonant transfer operation.
  • the design of an open-circuit filter i.e. terminated by a resistance such as R on one side but with an opencircuit on the other side, can be considerably simplified by using the technique of the reference filter described in the IRE Transactions on Circuit Theory, vol. CTS, December 1958, pp. 236-252, Recent developments in filter theory, V. Belevitch.
  • the reference filter is a filter conventionally terminated on both sides and with an effective transfer function equal to the voltage transfer function of the open-circuit filter. Simple relations can then be shown to exist between the image impedances of the two filters and the open-circuit filter may thus be realized by known synthesis methods.
  • An open-circuit filter may be designed in this Way from the reference filter as a ladder structure which may be called the intermediate filter because it has a last series branch on the open-circuit side, i.e. on the side of T (FIG. 2), which obviously can be suppressed to finally reach the open-circuit filter design whose structure thus ends wit a shunt branch, i.e. C on the open-circuit side.
  • A shows a peak at that point defined by
  • These frequencies x, and x will be defined as the practical lower and upper cut-off frequencies.
  • the attenuation A may thus be identified with the maximum attenuation allowable in the passband while x, and x will correspond to the practical cut-off frequencies given to the designer.
  • FIG. 4 gives the envelope A i.e. an upper bound, of A, at x, and x the attenuation may be smaller than A
  • the values x and x are thus determined by equating (5) with (6) which, by using (1), gives:
  • the band-pass filter having the structure of BPF in FIG. 2 can be shown to be an antimetrical band-pass filter exhibiting an attenuation pole at h above the upper cut-off frequency and which is desirable in order to attenuate the sampling frequency.
  • the design values can be secured along the conventional Iines aIready explained and can be expressed in terms of the parameters a, b and m already defined as Well as in terms of a further dimensionless parameter m function of the infinite attenuation frequency f i.e.
  • the elements C L and L can be expressed by ab 1 f R 11) R 41rabf 12) iul 41raf (m.,+m1) wherein N is an auxiliary parameter given by
  • the remaining four elements of BPF involve the step-up ratio n of the ideal transformer because, for such an antimetrical bandpass basis, the ideal transformer is usually located at the open end of the structure, i.e. between C and T, and not next to L as shown in FIG. 2 to enable its incorporation, together with L L into a practical inductive coupling.
  • Capacitor C is given y wherein N is a second auxiliary parameter given by 2 o+ 1) o 1) 1( o
  • N is a second auxiliary parameter given by 2 o+ 1) o 1) 1( o
  • the remaining elements are given indirectly by This would normally have only half this value and in practice it would then be close to unity, but apart from the ideal transformer part of the antimetrical band-pass filter design, it is desirable to consider a further step-up ideal transformer before reaching the transistor gate T.
  • the peak current flowing through the transistor T during the resonant transfers should usually be limited to some acceptable value. With an harmonic resonant transfer, this current which is proportional to reaches a peak after one third of the transfer time, during which T is unblocked, has been reached. Hence, it is desirable to choose C as low as possible. But C corresponds to the capacitance seen into BPF from T at infinite frequency since the resonant transfer time is very short, e.g. two microseconds. Thus, the impedance seen into BPF, from the resonant transfer circuit is essentially the combined capacitance of C C and C such elements as L having too high an impedance to have any effect on the high frequency transfer operations.
  • n is given by If C is decreased to limit the transistor current this means that n will have to be increased in order to still match the resistance R to that part of the filter impedance on the right-hand side of the transformer.
  • Such an increase of n means however that for a given power to be transmitted from E, the voltage applied to the tran sistor will be increased together with n.
  • This is also a limiting condition since there is a maximum tolerable voltage for the transistor T.
  • a compromise must therefore be sought and for example with a maximum energy level of 6 mw. and a resistance R of 600 ohms, a further step-up of 2 towards a transistor 2N1170 used for T was found satisfactory, therefore giving the overall stepup n of Formula 21.
  • a symmetrical band-pass filter (not shown) also designed on an image parameter basis and in which the elements L and C are no longer present leads to a value for U corresponding to L given abR m +m which leads to much larger inductance than L +L which will constitute the primary inductance, on the side of R of transformer TR (FIG. 1).
  • Other drawbacks of such a symmetrical band-pass filter would be that without the elements L and C of FIG. 2 it cannot correspond to the structure of FIG. 1 involving physically realizable coupled inductances and a DC. blocking capacitor.
  • practical realization would inevitably bring in spurious elements, i.e. leakage inductance of the transformer and the DC. blocking capacitor which can never be of ideally negligible impedances.
  • L L +L
  • L L M and L of FIG. 5 have been indicated above instead of L L and n of FIG. 2, since they are those which will be used in practice.
  • L is much smaller than that which would be necessary if, as is usually the case, the transformer must try to approximate ideal conditions. This means that for an equal size the resistance of the windings will be much smaller thereby substantially reducing the transmission losses in the passband.
  • the value of L is also considerably smaller than that of L' given by (23) which, for an equal size of the coil would lead to a Winding resistance approximately 12 times greater.
  • FIG. 6 shows attenuation curves comparing the design of FIGS. 2 and with one on the lines of FIG. 1 using a transformer of higher inductance value.
  • the element values of the design on the lines of FIG. 1 are C 25,000 pf.
  • the curves correspond to practical measurements on a time division multiplex connection such as shown in FIG. 1 and 1 represents the attenuation with the higher L value while 2 shows that with the new design (L is around 50 mh.).
  • L is around 50 mh.
  • a much flatter passband response is secured together with higher attenuation and a much steeper transition between the pass and stop bands.
  • the inductance of the transformer corresponding to response curve 2 is now small enough that a ferrite pot core cou d be used instead of Permalloy laminations, as we l as a greater winding wire cross-section. Hence, this leads not only to a diminution of the winding resistance and of the losses but also to a cost price reduction.
  • Curves 3 and 4 (dashes) give the levels of the side band signals corresponding to curves 1 and 2 respectively.
  • An impedance network for an electrical transmission system wherein the. signals are transmitted by means of pulses repeated at a sampling frequency, means for resistively terminating said network, coupled inductance means for coupling said impedance network on one side to said resistive termination, means for coupling said net: Work on the other side to a gate regularly unblocked at said frequency wherein the improvement comprises said coupled inductance means including a shunt inductive impedance in the transmission path whose value at the lowest frequency to be transmitted and when measured on the side of said resistive termination is lower than three times the impedance of said resistive termination.
  • said band-pass filter is antimetrical and comprises: a series resonant circuit on the side of said resistive termination, followed by a shunt anti-resonant circuit, by a series antiresonant circuit and by a shunt capacitance coupled to said gate through a transformer.
  • series inductance means are associated with said transformer means and located between said transformer means and the capacitance part of said shunt anti-resonant circuit.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
  • Filters And Equalizers (AREA)
  • Networks Using Active Elements (AREA)
US624530A 1966-03-21 1967-03-20 Impedance network for resonant transfer multiplexing Expired - Lifetime US3505477A (en)

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BE (1) BE695739A (forum.php)
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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114812622A (zh) * 2022-05-20 2022-07-29 电子科技大学 一种基于三阶例外点的高灵敏电路

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2081861A (en) * 1935-06-10 1937-05-25 Hazeltine Corp Band-pass filter
US3187101A (en) * 1959-10-20 1965-06-01 Int Standard Electric Corp Time division multiplex resonant transfer system

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2081861A (en) * 1935-06-10 1937-05-25 Hazeltine Corp Band-pass filter
US3187101A (en) * 1959-10-20 1965-06-01 Int Standard Electric Corp Time division multiplex resonant transfer system

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114812622A (zh) * 2022-05-20 2022-07-29 电子科技大学 一种基于三阶例外点的高灵敏电路

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SE336853B (forum.php) 1971-07-19
BE695739A (forum.php) 1967-09-20
CH489156A (de) 1970-04-15
GB1178493A (en) 1970-01-21

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